Volumetric physiological measuring system and method

ABSTRACT

A system and method for measuring volumes and areas using electromagnetic induction techniques. A current is generated and fed into one of two coil assemblies to induce voltage into another coil assembly to provide accurate values for volume or area.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.09/428,358 filed Oct. 28, 1999, now U.S. Pat. No. 6,374,667 which isincorporated herein by reference, and claims benefit U.S. ProvisionalApplication No. 60/106,282 filed Oct. 30, 1998.

FIELD OF THE INVENTION

The invention relates to a system and method for accurate measurementsof volume and area of objects, utilizing electromagnetic inductiontechniques.

BACKGROUND OF THE INVENTION

Numerous opportunities exist in which accurate measurements of volumeand area are essential, particularly of objects or portions of objectshaving variable volumes or areas. For example, in the field of medicine,the recording of breathing volumes in patients is often quite critical.Unfortunately these measurements are often crude and inaccurate, or atbest rely upon outmoded technical modalities or upon unreliable. Therecording of breathing volumes in patients is currently either performedby connecting a volume flow-sensing device to a subject's airway (e.g.by use of a spirometer or tachymeter) or by measuring the mechanicalexcursions of the chest and abdominal walls. For long-term monitoringpurposes, the airway-based techniques are inappropriate since theyinterfere with normal breathing and are unpleasant for the patient. Thisis particularly relevant to children and certain other patients.Although airway-based techniques are currently used in patientsdependent on respiratory-assist devices there may be less intrusive andmore reliable means of obtaining such data.

Similarly, techniques that rely on recordings of chest and abdominalwall movements are either strain gauge based (recording of changes inbody circumference length), or based on elastic inductive electricalconductor loops arranged around the chest and abdomen of the patient.Recordings of the inductance of the loops can then be used to estimatethe magnitude of cross sectional area variations of the chest andabdominal compartments. U.S. Pat. No. 4,308,872 is an example of thisself-inductance loop estimation technology. Such methods might be usedfor quantitative measurements of respiratory volumes only after acalibration procedure where the patient breathes known air volumes withvariable respiration movement distributions between the chest andabdominal compartments.

Currently, most devices for measurements of chest volume variations haveshortcomings related to calibration, stability, accuracy or reliability.The methods are either based on measurements of circumference of thechest and abdomen (strain-gauge transducers), or on measurements of theelectrical inductance of conductor loops arranged around the chest andabdomen. The reason for using abdominal sensors is that the downwardmovement of the diaphragm during inspiration causes volume changes ofboth the chest and abdomen that have to be added to estimate the lungvolume excursions. There is no fixed ratio between the abdominal andchest volume variations. Indeed, the relative contributions to totalvolume variations might even vary as a consequence of respiratoryeffort, airway resistance or sleep state. Thus, independent calibrationsof both measurement sites are necessary (when using known techniques) toestimate the actual volume variations caused by respiration.

The strain gauge or circumferential distance methods have no simple orreproducible relation between the measured variations and the volumesthat are measured. This relation depends on assumptions about therelation between the area enclosed by the loop and the length of theloop that are valid only for a fixed geometry. Although some of themethods based on inductance may claim that area is measured (i.e., it isassumed to be proportional to loop inductance), the assumption is onlyvalid as long as the relative shape of the loop is conserved.Unfortunately, this is not the case for the cross-sectional areavariations of the human chest or abdomen that are caused by respiration.

Common to all area-based methods is a considerable uncertainty in thecalculations of volume variations from the estimated area variations.One reason for this is that only point samples of the area (at the sitesof the transducers) are measured. To relate area variations to volumes,some means of calibration is necessary. The calibration proceduredepends highly upon patient cooperation to obtain coefficients for bothchest and abdominal measurement sites, and is virtually inapplicable tosmall children, patients with dyspnea (shortness of breath), andunconscious patients.

This invention describes novel systems and methods for volume and areameasurements, based on electrical inductance, in ways which eliminatethe shortcomings discussed above. The invention does not depend onpatient cooperation for calibration and is a true volume or areameasurement method that does not depend on assumptions about therelations between circumference, area and volume.

SUMMARY OF THE INVENTION

A new family or class of devices is provided for recording ofcross-sectional areas and volumes of objects, in particular portions ofthe human body. This includes static measurements as well asmeasurements of the smaller overlying variations in area and volumecaused by respiratory and cardiac activity. The principle relies onmeasurement of electromagnetic induction between one or more electricalconductor loops wrapped around a body part to be measured, and one ormore remotely located electromagnetic coil arrangements. By properdesign of the coils, the induced voltages that are measured will beproportional to the area or volume that is measured.

In one embodiment, a device is provided for measuring the movement of anobject which comprises means for creating time-varying magnetic fieldsat least large enough to surround the object. Electrical circuits areadapted to conform to the surface of the object, and voltage monitoringmeans are connected to the electrical circuits, whereby motion of thesurface creates a measurable change in induced voltage in the circuitsthat correlates to the movement of the object.

In another embodiment, a method is provided for measuring the area orvolume of an object which comprises the steps of adapting electricalcircuits to conform to the object surface, and measuring the voltageinduced in the circuits by a time-varying magnetic field surrounding theobject, whereby the change in the area or volume of the object may becalculated without calibration of the device to the individual.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an arrangement of serially connected loops and a magneticfield vector.

FIG. 2 is a block diagram of a first embodiment of the system of theinvention.

FIG. 3 is one embodiment of a transducer design for use as a coil means.

FIG. 4 is a sample recording from the use of the system of FIG. 2.

FIG. 5 is a magnified view of the raw volume curve of FIG. 4,demonstrating cardiac pulsations in volume.

FIG. 6 is a schematic view of a second embodiment of the invention.

FIG. 7 is a schematic view of a third embodiment of the invention.

FIG. 8 is a schematic view of a fourth embodiment of the invention.

FIG. 9 is a schematic view of a fifth embodiment of the invention.

FIG. 10 is a schematic view of a sixth embodiment of the invention.

FIG. 11 is a schematic view of a seventh embodiment of the invention.

FIG. 12 is a screen view graph of simultaneous recordings using aspirometer and the system of the invention.

FIG. 13 is a screen view graph of blood volume variations in the chestand abdomen caused by cardiac activity.

FIG. 14 is a block diagram schematic view of one embodiment of thesystem of the invention.

DETAILED DESCRIPTION OF THE INVENTION

By way of technical background, and in reference to FIG. 1, it may beseen that the induced voltage in a closed electrical conductor loop 10perpendicular to a homogeneous magnetic field that varies with time is:U _(loop)=(dB/dt)A _(loop)  Eq. 1)where B is the magnetic field strength, and A_(loop) is the area of theloop regardless of its shape and t is time.

Thus, when a homogeneous magnetic field with a known intensity andtemporal variation and with 1 direction that is along a patient's bodyaxis is generated, measurements of induced voltages in conductor loopswrapped around the patient's body will give accurate area measurements.In order to measure volume, it is necessary to add together the volumesof a stack of slices, each with a measured area and a known thickness.This is readily accomplished by using several conductor loops wrappedaround the patient's body. If the spacing between the loops is constantand known, then the volume to be measured (V_(body)) may be expressedas: $\begin{matrix}{V_{body} = {{s\quad\lim{\sum\limits_{s\rightarrow 0}A_{loop}}} = {{s\left( {{\mathbb{d}t}/{\mathbb{d}B}} \right)}\quad\lim{\sum\limits_{s\rightarrow 0}U_{loop}}}}} & \left. {{Eq}.\quad 2} \right)\end{matrix}$where s is the spacing between the loops.

Summation of the induced voltages is accomplished by electrical serialconnection of the loops, effectively reducing the complexity of thesystem, since only one voltage needs to be measured. This is animportant advantage of the invention disclosed herein resulting inreduced system complexity. The choice of the loop spacing s (alsoreferred interchangeably herein as distance d) is a compromise betweenaccuracy and convenience in the design of the volume transducer. A lowvalue of s means that a higher number of loops will be needed toencompass the measurement region, which typically is from the level ofthe axilla and down to the crest of the hips, when measuring a torso. Ifthe number of loops becomes high, the self-inductance of the seriallyconnected loops starts to increase dramatically, with implications forthe design of the electronic circuits. A preferable embodiment is to usevalues of s in the range of four to eight cm, which are the smallestvalues to be used for infants. This typically results in eight (8) toten (10) loops. Other configurations, however, are possible. Themagnetic field need only be homogeneous within the volume occupied bythe relevant parts of the patient's body or other object being measured.Practical experience indicates that a field of sufficient intensity andhomogeneity can be generated just by arranging a conductor wire as avertical rectangular loop, following the floor, walls and ceiling of theroom, and feeding an alternating current through the loop. Preferablevalues are 10 kHz to 200 kHz with currents below 1 A, although otherconfigurations are possible. For example, more than one electricalconductor loop may be used.

FIGS. 2-8 relate to a first embodiment of this invention discussedbelow. This embodiment, shown in block diagram form in FIG. 2, comprisesintegrating both the alternating current source for the current loopthat generates the magnetic field and the circuits needed to amplify andrectify the returned signal into a single unit. Stable crystaloscillator 30 controls system 20, with the output frequency of theoscillator being divided down to the desired value by binary counter 34.The suggested values of 1 MHz and a division of 16 are arbitrary. It is,however, an advantage that the signal fed into the power amplifier 36 isa symmetric square wave, since this waveform lacks even frequencyharmonics. The removal of unwanted harmonic frequency components byfilter 37 is then easier. Such removal of harmonics is preferable toavoid radiation interference with other electronic equipment. Theamplifier output will eventually be fed into an inductive load thatabsorbs almost no energy. By using a (class D) switching amplifierdesign, and by proper design of the low-pass filter, the total powerconsumption of the amplifier unit can be minimized, reducing the demandsfor circuit cooling and for power supply capacity. The low-pass filter37 would likely be a passive one, and would not introduce substantialphase-shifts to the voltage waveform at the frequency used. The signalpicked up by the serially-connected loops, which in one embodiment arewrapped around a patient as shown and described below in relation toFIG. 6, is first fed into band pass filter 44. The purpose of thisfilter is to increase the system noise immunity. The filter can bedesigned as a narrow-band resonant transformer, effectively implementinggalvanic separation of the conductive wires close to the patient and therest of the apparatus.

Pre-amplifier 48 is useful since the noise performance of the amplifiersis better than that of demodulator 53. The gain of the amplifier neednot be more than necessary to suppress the noise of the demodulator,with a typical value of about 20 to 25 dB.

The coherent demodulator 53 rectifies the alternating voltage signal,and is easily implemented by CMOS analog switches, such as the CD4016 orCD4066 circuit. Using a coherent demodulator has two advantages. It isvery linear over a broad signal intensity range, and together withlow-pass filter 58, a substantial improvement in relation to noisesuppression is achieved. The effective bandwidth of the amplifier chainwill be twice the cut-off frequency of low-pass filter 58, with oneexample of atypical value of this cut-off frequency being 5 Hz to 15 Hz,with the highest value suited for infants with a higher naturalrespiration rate.

The signal-conditioning block 63 serves several purposes. It providesamplification of the signal, making it suited for further processing ina computer-based system. Also, the unit provides some means of coarseadjustment of the amplification, preferably controlled by the analyzingsoftware. Since the signal at the output of the low-pass filter 58contains a substantial DC or constant component (corresponding to thetotal volume of the body inside the coils), some means of subtractingthis component will increase the sensitivity of the system in detectingsmall volume changes. Such subtraction can be made with an electronichigh-pass filter, the cut-off frequency of which must be substantiallybelow common respiration frequencies (0.05 Hz or below). In order toshorten the time for the output signal to stabilize at start-up of thesystem, a means of shortening the time constant of this filter (byresistor switching) is desired. In some cases, where slow changes notrelated to respiration are measured, absolute readouts of volume withoutany zero subtraction might be desired. Applications of this might bemonitoring of edema development, vascular volume reflexes, muscle growthor atrophy, effects of surgical procedures, etc.

The electronic circuits described above are easily constructed withinless than 50 cm² of circuit board area, and could be integrated with anA/D converter and interface control circuitry on a plug-in board for anordinary personal computer, for example as shown in FIG. 14 below. Acomplete stand-alone unit could also be made, with the size andcomplexity being dependent on the amount of signal analysis,presentation capability and storage capacity built into the unit. Itshould be emphasized that the above illustrative description is for onlyone embodiment of the invention, with numerous alternate means ofpracticing the underlying technology also being within the scope of theappended claims. Parts of the function of the suggested block diagram inFIG. 2 could be implemented using digital signal processing or othermeans. At least one alternate construction is to let all processing ofthe received signal be digital from and including demodulator 53 with asampling rate of exactly twice the loop current frequency.

The transducers containing the current loops can be made with varyingmaterials, such as copper or similarly conductive metalization, as wellas other designs. For example, one design of elastic electricalconductors might integrate a spring component with the electricalconductor. Also, ease and costs of manufacturing are important. Anothersolution is to develop a disposable unit, based on metallized plasticbands as the conductor elements. Since the induced voltages are very low(in the order of a few millivolts), and the impedances are low (unlessthe number of loops becomes high), there may be no need to use insulatedconductors. Indeed, the magnetic field generated by a room coil isweaker than that generated by the deflection magnets in a televisionset. Prolonged exposure to such fields is generally deemed to beharmless. Since a pick-up conductor coil wrapped around the patient canbe galvanically separated from the rest of the equipment (as is alreadydone in most other electrical equipment used for patient monitoring),there will not be any electrical shock hazards either.

One embodiment for the design of a disposable transducer is shown inFIG. 3. Strips of metallized foils 72 and plain plastic foils 74,possibly one cm wide, are made into a meshwork by welding at the crosssections. Alternatively, the whole assembly can be stamped from a sheetof foil with metallized bands on it. The thickness of the foil isselected so that the zig-zagged strips have a suitable elasticity. Arectangular sheet of such meshwork is wrapped around the patient or Isubject, with a skew, so that the conductor strips are turned into ahelix. The seam must provide electrical conducting, either by using somesort of clips, or simply by twisting the pairs of strips together. Acable may be connected to the recording apparatus. It is recognized thatother devices may be used to arrange the coil configurations around theobject to be measured, for example, as will be discussed below.

One embodiment of the system according to FIG. 2, with some minormodification, was tested using the following parameters:

-   -   Current frequency: 70 KHz    -   Loop current: 200 mA (a 4-turn loop was used, thus the effective        current was 800 mA)    -   Loop size: 2×2 meters    -   Magnetic field strength in patient body: ≈2 μT    -   Incremental volume sensitivity (system noise level): 1 ml

This embodiment deviates somewhat from the suggested diagram in FIG. 2.A free-running RC-oscillator was used as the frequency control element,and the low-pass filter between the power amplifier and the room loopwas omitted. More strict conformance with the block diagram in FIG. 2will probably result in even better performance, especially with regardto the noise level.

A sample recording from the use of this embodiment is shown in FIG. 4.The volume waveform 81 can be used to calculate breath-by-breath tidalvolumes in liters 77 by considering the amplitude of the waveform.Ventilation in liters per second 75 can be found by dividing the tidalvolume by the duration of the individual cycle. The instantaneousfrequency of respiration (not display) can be found by inverting theduration of the cycles. By careful inspection, variations are observedin the volume that occur in synchrony with the heart rate, and whichprobably reflect variations in thoracic and abdominal blood volumecaused by the beating action of the heart. A magnified view of the rawvolume curve 81 in FIG. 4 (from 61 to 64 seconds on the time axis) isshown in FIG. 5 to demonstrate cardiac pulsations in volume. Thesepulsations are, however, far smaller than the expected cardiac strokevolume of the subject, and reflect the net volume variations of summedabdominal and thoracic blood volume, which might be related to thefraction of the cardiac stroke volume that is diverted to the limbs andbrain. Clearly, this provides a valuable diagnostic indication byanalysis of this waveform, which may be used for monitoring andassessment of patients with heart failure, shunts or valvular disease.The waveform can easily be extracted from the underlying respiratoryvariations by coherent averaging, possibly using the patient's ECG as atiming reference. Volume minima coinciding with cardiac end-systole areindicated with arrow markers Ces. Thus, an apparatus combiningrespiratory and cardiac monitoring is disclosed. The curve trace of FIG.5 also demonstrates the noise level of the system which, when translatedto measured volume, is in the order of one ml RMS or standard deviation.

As noted above, a typical use of the invention is to continuouslymeasure the volume or area of the chest and abdomen of a human patient.This volume or area will show variations in synchrony with the movementsof breathing. These variations correspond to the changes in the volumeof gas contained in the lungs, since all fluids and tissues present inthe measured volume are incompressible. It is essential also to includethe abdomen in the measurement, since breathing action also comprises upand down contraction of the diaphragm muscle, which again causes inwardand outward motion of the abdominal walls. Analysis of the time courseof volume variations may be used for estimating the quantitative amountof breathing, and a plurality of variables describing the breathingpattern, such as frequency, depth of breath (tidal volume) and airwayflow are also discernible.

It should be noted that this invention occurred due to the recognitionof the shortcomings of the previously known technology for measuringvolumes and areas of objects having an outer material within which aninner volume may change. In addition to human torso and limbs, thisinvention is well suited for use in the industrial applications ofvarious types, such as where internal volumes may be fuel containers,propellants, or other materials having variable volumes or areas.Regardless of the application, the inventors have recognized the valueof the fact that the induced voltage in a conductor loop by ahomogeneous alternating magnetic field is exactly proportional to theloop area, independent of the loop shape. The induced voltage in suchloops will be added together when the loops are connected in series, forexample as when the voltages from individual battery cells are added ineveryday electrical equipment. This provides the basis for themeasurements of volumes by a single voltage reading from a stack ofloops with equidistant spacings.

In addition to greater accuracy of this system and method of obtainingmeasurements, this technique is also important due to its speed. Inparticular, the system of the invention does not need to be calibratedto each patient. Rather, volume and area measurements may be accuratelytaken regardless of the shape or variation pattern of the object/patientprovided that the conductive coil follows the contours of theobject's/patient's surface contours. For example, in one embodiment, apatient may be equipped with a tightly fitting elastic shirt or garment.This shirt has embedded electrical conductors that comprise a pluralityof closed circumferential loops that closely follow the body contours.Electromagnetic induction between this body coil and another remotelylocated fixed coil is then measured. This may be done by feeding analternating electrical current (with frequency in the range of about 10kHz-200 kHz) through either coil, and measuring the induced voltage inthe other coil. It is important to note that magnetic fields that areused in the invention are not affected or modified by the presence ofbody tissue, so the physical laws describing electromagnetic phenomenain void spaces are still valid.

In one embodiment of the invention, a passive use of the patient coil isused, for example as will be later discussed in relation to FIG. 6. Inthat embodiment the room coil 101 is generating the magnetic fieldrather than in a “reversed” arrangement where the patient coil isenergized instead of the remote or room coil. Such a reversedarrangement will be further described below in relation to FIG. 7. Thefirst embodiment mentioned above comprised a large (a few meters ormore) electrical conductor loop mounted along a building structure suchas a wall, floor, or ceiling in order not to be obstructive. However,another embodiment of this coil is possible and consists of a plurality,such as three, carefully positioned and matched small coils, as shown inFIG. 8. These coils will together generate the same type of magneticfield as a large coil would do at the patient measurement site. Theseembodiments for coil assemblies can be used instead of a bulkier roomcoil either as a magnetic field generator, or as a magnetic field sensorin the reversed coil arrangement. When the design of the overall systemuses this coil configuration as a sensor, it may be possible to usenon-inductive technology, such as point sensors or magnetometers, or thelike. In the embodiment of FIG. 7, instead of generating the magneticfield with the room coil, the patient coil is energized and used forgenerating a magnetic field whose properties depend on the volume orarea to be measured; while the room coil or three-coil assemblymentioned above is used for picking up an induced voltage caused by thefield. By proper design of the electronic circuits that energize thepatient coil (constant current amplifiers), the desired linear area orvolume dependency of the induced voltage will be preserved.

Another refinement of the system allows simultaneous measurement of twoportions of an object, such as a plurality of patient volumecompartments, typically the chest and abdomen. This gives supplementaryinformation about the distribution of muscular effort in breathing, andprovides information relevant for diagnosis of infant pulmonarydiseases, especially diseases associated with airways obstruction.Adding the two measured volumes will still give the total volumemeasurement advantages of the other embodiments. In order to performsimultaneous measurements, the patient coil is divided in two halves byan electrical “tap” connection at the center turn, as shown in FIG. 10.

Thus what has been provided by this invention is measurement of the areaof an electrical conductor loop by recording of the induced voltage inthe loop when the loop is positioned in a homogeneous magnetic field.The magnetic field must be alternating, such as a field generated by analternating electrical current. The area of the loop is, according tophysical laws, proportional to the induced voltage. Also, the inventionenables measurement of the sum of areas of multiple conductor loops byrecording the induced voltage either individually, or as a singlemeasurement when they are electrically connected in series. Measurementof the amount of volume contained inside a plurality of equidistantlystacked conductor loops, by measuring their summed area and multiplyingthis measured area with the distance between the loops to obtain volume,is also disclosed. The use of these techniques to measure crosssectional areas or volumes of parts of animal or human bodies, using asystem of one or more elastic and compliant conductor loops tightlywrapped around the body part, is quite advantageous. Further benefitsare taught by the systems and methods herein for achieving accuratemeasurements of variations in the volume of an animal or human body withthe above techniques in order to record breathing, and measurements ofvariations in the volume of an animal or human body with the abovetechniques in order to record volume variations caused by cardiacactivity. Additional embodiments of the invention are disclosed whichprovide the further advantages of reversal of electrical coilconnections; more compact design for the room coil, comprising two ormore smaller coils with optimized shapes and positions; an arrangementcomprising three small rod-like coils wound on ferrite cores arrangedalong a straight line, with optimized positions and signal intensityweighting to generate a homogeneous magnetic field or sensing volumeencompassing the patients body or body parts; and simultaneousmeasurement of two or more areas or volumes by phase, frequency or timemultiplexing, using any of the techniques above.

The inventors have determined that implementation of the followingembodiments require adherence to several principles. First, theelectrical conductor coils that are wrapped around the body part orobject to be measured must follow the body/object surface contours, evenwhen these contours change shape. Thus the coil assembly must be elasticand deformable. Second, for measurements of volume, the coils that arewrapped around the body/object part must be stacked with a known spacingbetween each loop. Coil loops stacked with a constant spacing might beconnected in series electrically in order to sum up their voltages andthus to ease volume calculations. Third, the remote coil or coils must,when energized by an electrical current, generate a magnetic field thatis reasonably homogeneous (of equal magnetic field strength) at thelocation of the body part to be measured. Only the vector component ofthe magnetic field that is perpendicular to the coils that are wrappedaround the body needs to be considered regarding this homogeneity. It isfurther desirable that the remote coils must be able to record the fieldfrom a point-sized magnetic dipole located inside the volume to bemeasured (i.e. the patients body) with a sensitivity that is independentof the location of the dipole, as long as the dipole is orientedperpendicular to the patient body coil loops. It is further desirable touse the remote coil or coils to generate a magnetic field (by feeding analternating electrical current through them), and then recording theinduced electrical voltages in the coils wrapped around the body part tobe measured. The voltage will be proportional to the area or volumeenclosed by the coils. The remote coil or coils might be arranged in amanner that meets the homogeneity criterion noted above. Finally, it isdesirable to feed an alternating current through the stacked coilswrapped around the body part to be measured and then to measure themagnetic field generated by this current. This can be done by recordingthe induced voltages in the remotely located coil or coils. The voltagewill be accurately proportional to the area or volume enclosed by thepatient body coils. During these measurements it is essential to keepthe current in the patient coil constant even if the self-inductance ofthis coil varies due to dynamic variations in shape. Thus, aconstant-current circuit, or equivalent means, must be used forenergizing the patient coil. The design of such a circuit is trivial forthose skilled in the art.

Several embodiments of the system and methods of the invention aredisclosed in FIGS. 6-11. In FIG. 6, an arrangement is shown in which aconductive coil means 107 is configured tightly about the variouscircumferences of a torso portion of a human 110. Fixed coil means isremotely located relative to the conductive coil means about the human.Current, generating means is provided and generally shown as signalgenerator 117 and power amplifier 123 to create an induced voltage inthe conductive coil means which changes over time due to the respirationof the human 110. The induced voltage in the conductive coil means isthen amplified by amplifier means 127, and rectified by rectifier means133, and provided as a volume output that is used for measurement ofarea or volume. The system and method embodiment of FIG. 7 is similar tothose of FIG. 6 except that in FIG. 7 the conductive coil means 107 isenergized by a constant current, and the induced voltage in the fixedcoil means or room coil is used for measurement of area or volume. Asnoted above, a constant current amplifier means 136, or equivalentcircuit component, is provided in this embodiment.

FIGS. 8 and 9 illustrate the use of a more compact three-coil systemdesign for the fixed coil means instead of the large room coilconfiguration of earlier embodiments. The individual coils 144 might beserially connected as shown in FIGS. 8 and 9, or they might also beconnected in parallel if this is considered during their design.Alternatively, magnetic field sensing devices not based onelectromagnetic induction could be used, such as sensors based on thewell known Hall effect. Arrangement of a plurality of small coils abovethe object being measured is designed to generate a homogeneous magneticfield. The coils are typically wound on ferrite rod cores and each coilwill then generate a magnetic field pattern similar to that of amagnetic dipole. The positions and number of turns of the coil loops areoptimized to satisfy the homogeneity criterion noted above. The serialconnection assures identical currents in all three coils. The individualfield strength contributions of the coils might be set by their numberof loops, physical dimensions or magnetic core material permeability.One example of a three-coil system includes a small cylindrical coil,having about a 10 cm length and 1 cm in diameter. This will generate amagnetic field that closely resembles that of two opposite magneticmonopoles at the two coil ends. The field can be strengthenedsubstantially by inserting a rod made of a material with high magneticpermeability into the coil. A suitable material is ferrite, which issimilar to the core material in electronic high-frequency transformers.The remote magnetic field from such a device is close to that of anideal magnetic dipole, which can be described in exact mathematicalterms. The process of finding an optimized design for a homogeneousfield is initiated by deciding a set of physical constraints, such asthe desired shape and extension of the spatial volume where ahomogeneous field is desired, and the space into which a given number ofsuch coils are to be positioned. A set of parameters or degrees offreedom must also be defined, with typical parameters being coordinatesfor the position and angular orientation of the individual coils, andthe strength of the field generated by the individual coils. A generalnumeric error minimalization routine such as the Levenberg-Marquardmethod is then used to adjust the parameters to find a solution to theproblem where the field homogeneity is maximized. The three-coil designdescribed herein has been developed with this method.

Systems according to these embodiments also comprise sensing and controlmeans for controlling operation of the system. Timing and multiplexswitching means may also be included for providing simultaneousmeasurement capabilities of multiple portions of the object beingmeasured, for example a thoracic region and an abdominal region, orperhaps a plurality of limbs. One of the methods of performing aplurality of simultaneous volume or area measurements is to use phaseencoding of the signals. The scheme can be adapted to systems where thepatient coil is energized, and the room coil is used for sensing thefield, as shown in FIG. 10. The two constant current amplifiers 136 aredriven by signals that are phase shifted 90° at phase shift means 158with respect to each other. The patient coil or conductive coil means107 is equipped with a center-tap connection means, so that the twohalves can be energized by independent currents. The two demodulators169 rectify the signal that has been picked up by the three room coils144 and amplified. Since the demodulators receive reference signals thatare phase-shifted in the same manner as the inputs to the currentamplifiers, they will respond to signal components that match the phaseof the two patient coil signals. The apparent swap of output signals(the demodulator that receives the same phase as the abdomen coilcurrent gives an output that reflects chest volume) is caused by aninherent 90° phase shift of the induced voltage in the sensing roomcoils compared to the current-dependent magnetic field strength from thepatient coils. It is recognized that various techniques might be used toperform simultaneous measurements of two or more volumes or areas. Ifthe room coil generates the magnetic field, then independent signalprocessing chains consisting of amplifiers and rectifiers as shown inother figures can be connected to any number of coils wrapped around thepatient's body. For the coil arrangement indicated in FIG. 10, thedifferent patient coils 107 might be driven with currents of differentfrequencies, and electronic band-pass filters might be used to extractthe individual volume signals from a common room coil and pre-amplifier.Time multiplexing might also be used, energizing the patient coils in arapidly repeating sequence, typically cycling through all coils between10 and 1000 times per second, and sampling a common volume output signalin synchrony with this sequence to separate the volume measurements fromeach other. An additional potentially useful technique for simultaneousmeasurements of several volumes might be to use different,non-correlated pseudo-random bit sequences for generating patterns ofcurrent variation in two or more patient coils. The voltage picked up bythe room coil is amplified, and the individual volume signals arerecovered by demodulating the composite signal using the correspondingbit sequences as reference signals. Due to the encrypted nature ofpseudo-random bit sequences, this type of instrument is expected to bevery immune against electromagnetic interference noise, and instrumentslocated close to each other will not interfere with each other'soperation.

A measurement system as described above, with any of the disclosed roomcoil designs or electrical connection arrangements needs to be initiallycalibrated but not calibrated at every patient or object measurementsession. This calibration will compensate for variations in: the currentused for generating the magnetic field, the exact geometricalarrangement of the room coil with respect to the patient's position, andthe gain of the electronic amplifier and characteristics of therectifier circuit calibration is performed by connecting a referencecoil instead of the patient coil, and positioning this reference coil inthe same position as the patient's body will be later duringmeasurements. The summed cross-sectional area of the loops of thereference coil are known from the geometry and number of turns of thiscoil. Typically, this area should be of the same order of magnitude asthe area of the patient coil. A constant, k_(c) is then calculated as:k _(c) =a _(c) /U _(c)  Eq.3)where a_(c) is the area of the reference coil, and U_(c) is the voltagereading of the volume output signal.

For measurements of area, using a single conductor loop wrapped aroundthe relevant body part, the area A can be calculated as:A=U _(c) ·k _(c)  Eq. 4)where U is the measured voltage.During volume measurements, using serially connected equidistantlyspaced loops the instantaneous volume V is calculated from the voltagereading U as:V=U·d·k _(c)  Eq. 5)where d is the spacing between the loops of the patient coil.

EXAMPLE 1

A circular electrical conductor loop with a radius of 1.5 meters wasplaced on the floor. An alternating current of 0.1 A with a frequency of100 kHz was fed through the conductor from a signal generator. A small,10×10 cm (100 cm²) square conductor loop was connected to the input of abroadband signal amplifier with a gain of 40 dB, and the output fromthis amplifier was passed through a narrow 100 kHz band-pass filter withno gain. The output signal from the filter was presented on anoscilloscope screen, where voltages could be read by cursors. When thesmall loop was positioned flat on the floor in the center of the largercurrent loop, a voltage of 5 mV p/p was measured. This voltage remainedessentially constant when the small loop was elevated 50 cm above thefloor, and it also remained constant when the small loop was moved up to50 cm in any horizontal direction away from the center of the largecurrent loop. Another square loop, measuring 14.1×14.1 cm, with an areaof 200 cm² was then connected instead of the 10×10 cm loop. The voltagereading was now 10 mV.

EXAMPLE 2

The same conductor loops as described in Example 1 were used, but the0.1 A current from the signal generator was fed to the 100 cm loop, andthe large loop on the floor was connected to the input of the 40 dBbroadband amplifier. The small loop was positioned as explained inExample 1. The same general relations between induced voltage and loopposition and area was observed as in Example 1, however the voltageswere reduced to 250 μV when using the 100 cm² loop, and 500 pV whenusing the 200 cm² loop.

EXAMPLE 3

Referring to FIG. 11, three identical cylindrical ferrite rods 175 witha diameter of 8 mm, length of 150 mm and permeability of 100 werearranged along a straight horizontal line with a common axis. Theend-to-end horizontal spacing between each rod was 50 cm. The two outercoils were equipped with 163 turns of lacquer-insulated 0.1 mm copperwire evenly distributed along a central 8 cm segment of the rod, whilethe center rod had 100 turns arranged in the same manner. The coils wereelectrically connected in series, and care was taken to assure identicalcurrent directions in all coils. The serially connected coils were thentuned to parallel resonance at 100 kHz with a common capacitor 177 ofabout 10 nF. An oscilloscope directly monitored the signal from theresonant circuit. A 10 cm×10 cm current loop carrying a 100 kHzalternating current of 0.1 A was positioned adjacent to the coilassembly, keeping the orientation of the loop perpendicular to theferrite rods common axis. It was found that the induced voltage in theferrite rod circuit varied only 5% from an average value for allpositions of the probing coil in the homogeneous region 182 indicated inFIG. 11.

EXAMPLE 4

The circular room coil described in Example 1 was positionedhorizontally 120 cm above the floor. An alternating current of 0.1 A at100 kHz was fed through the coil from a signal generator. A human wasdressed with an elastic Lycra textile shirt that covered the torso fromthe armpits and down to the ridge of the hip. A helical electricalconductor was attached to the surface of the textile shirt, making atotal of 10 equidistant turns around the torso. The spacing between theturns was 5 cm. The conductor was arranged in a fine zigzag pattern toallow for circumferential variations without becoming over-stretched orloosing contact with the body contour. The subject was positionedstanding with the chest and abdomen located close to the center of theroom coil and the amplitude of the induced voltage in the helical coilwas measured to be 350 mV peak to peak with an oscilloscope. Combiningthis with the reference measurements described in Example 1, the volumeof the torso could be calculated as:${350\quad{{mV} \cdot 5}\quad{{cm} \cdot \frac{100\quad{cm}^{2}}{5\quad{mV}}}} = {{35000\quad{cm}^{3}} = {35\quad{liters}}}$

EXAMPLE 5

The same subject and as in Example 4 wearing the same elastic textileshirt with electrical conductors is used. The subject is positionedsupine on a bench, and three small ferrite coils as in Example 3 aremounted 90 cm above the surface where the subject is lying, parallel tothe subjects body axis and centered above the subject's chest andabdomen. A constant alternating current of 0.1 A is fed through thecoil, which follows the surface contours of the torso. The voltage thatis induced in the ferrite coils is measured with an oscilloscope. Thisvoltage is compared to the voltage that is measured when a 100 cm² coilcarrying 0.1 A is positioned at the same site as the subject's torso,and a volume very similar to that described in Example 4 is calculated.

EXAMPLE 6

The experiment of Example 4 was repeated. The induced voltage from thepatient coil was rectified in order to generate a DC voltage signalreflecting instantaneous torso volume. This signal was sampled with 14bits of resolution by an analog to digital converter, and thentransferred to a computer at a sampling rate of 25 Hz. Equation 5 abovewas used to convert the voltage readings to volume values. The signalwas then fed through a digital high-pass filter with a cut-off frequencyof 0.05 Hz. This removed the time-averaged volume component from thesignal. The filtered signal then reflected breathing activity. Thesubject's breathing activity was also recorded by connecting the patientairways to a recording dry-seal spirometer by the use of a mouthpiece. Asimultaneous recording of breathing by the two methods is displayed inFIG. 12. As shown, the comparison of the simultaneous recordings ofbreathing volume by a dry-seal spirometer connected to a human subject'sairways (broken line), and the measurements using the device describedin Example 4 (solid line) are virtually identical. Upward deviations ofthe traces mean increasing lung air content. To ease the comparison oftraces, the spirometer trace has been inverted. The deviation betweenthe traces during the first four seconds is probably related to thermalequilibration within the closed spirometer system.

EXAMPLE 7

The experiment in Example 6 was repeated. Volume measured by theinductive device was recorded for 8 seconds while the subject held hisbreath in expiration phase. Variations in volume with amplitude of about15 ml occurring in synchrony with his heartbeats could be seen, as shownin line 188 of FIG. 13. These variations are expected to represent thetotal blood volume variations in the chest and abdomen caused by thebeating action of the heart.

While the above Examples illustrate some of the capabilities of theinvention, they are not intended to limit the scope of this disclosure.Also, as referenced above, various system embodiments are contemplated.FIG. 14 discloses yet another embodiment of the measurement systemsdisclosed herein. Fixed coil means 101 is configured as a sensor forinduced voltage created when current is generated in signal generator117 and current amplifier 123 and fed to conductive coil means 107within homogeneous region 182. The sensed voltage is amplified atamplifier 127, and is fed through rectifier 133, A/D converter 192, andbecomes the measured value 195 that transmits to personal computingand/or display means 201. Finally, status and control means providessignals 197 to give status and control commands.

It is recognized that the functionality and display of the accuratevalues being measured by the systems and methods disclosed hereinpresent significant advantages to users of the invention. The speed andaccuracy greatly enhances the value of multiple, real-time traces,traces with rapid x-axis and waveform details, traces with a wide rangeof time axis span of minutes to hours, and reconstruction displays.Numerous other display features and diagnostics are enabled due to thesystem advantages identified above.

It will be understood that various modifications and alternateconfigurations may be made in the preferred embodiments shown anddescribed herein without departure from the scope of the invention, andthat the invention embraces all such changes, modifications, andequivalents as are within the scope of the following claims.

1. A device for measuring the changing area within at least one portionof a mammal due to cardiac function, comprising: a. conductive coilmeans configured tightly about the various circumferences of at leastone portion of the mammal; b. fixed coil means remotely located relativeto the conductive coil means about the mammal; and, c. currentgenerating means for selectively providing constant alternating currentto either one of the conductive coil means or the fixed coil means tocreate an induced voltage in the other coil means representative of truearea within the coil means that is configured tightly about the variouscircumferences of the mammal portion or portions, within the signals andarea changing over time due to the cardiac function of the mammal, saidconstant current substantially increasing the accuracy of saidmeasurement system.
 2. The device of claim 1 in which the currentgenerating means generates current with a frequency range of about 10kHz to about 200 kHz and from about 1 milliampere to about 1 ampere. 3.The device of claim 1 in which the conductive coil means comprises aplurality of coil loops which are configured tightly about the thoracicregion and the abdominal region of the mammal.
 4. The device of claim 1further comprising sensing and control means for controlling operationof the device, the sensing and control means being electricallyconnected to the conductive coil means, the fixed coil means, and thecurrent generating means.
 5. The device of claim 4 in which the sensingand control means comprises timing multiplex switching means forproviding simultaneous volume measurements of both a thoracic region andan abdominal region of the mammal.
 6. The device of claim 5 in which thesensing and control means comprises multiplexing means for providingsimultaneous measurement of a plurality of regions of the mammalutilizing either phase, frequency or time multiplexing.
 7. The device ofclaim 1, further including a sensing and control means comprising anamplifier and a rectifier electrically connected to the coil means thatis receiving the induced voltage from the other coil means.
 8. Thedevice of claim 1 in which the fixed coil means comprises a plurality ofsmall coil elements that are configured for matching and positioning topermit the fixed coil means to generate a homogeneous magnetic fieldsimilar to a single large coil for either sensing an induced voltagefrom the conductive coil means or for generating a field to create aninduced voltage in the conductive coil means.
 9. The device of claim 8in which the plurality of small coil elements comprises three small coilelements.
 10. The device of claim 9 in which each of the small coilelements is wound on a ferrite core and arranged linearly with optimizedpositions and signal intensity weighting to generate a homogeneousmagnetic field at the portion of the mammal being measured.
 11. Thedevice of claim 1 in which the conductive coil means compriseselectrically conductive coil loops that are equally spaced and carriedby an elastic and conformable substrate that is suitable for wearing bythe mammal in a manner similar to a tightly fitting garment which isconfigured so that the coil loops always conform to the same surface ofthe portion of the mammal regardless of any shape change which thatportion of the mammal may experience during respiration.
 12. The deviceof claim 1 in which the conductive coil means comprises electricallyconductive coil loops that are spaced at constant and known intervalsand which are carried by an elastic and conformable substrate that issuitable for wearing by the mammal in a manner similar to a tightlyfitting garment which is configured so that the coil loops alwaysconform to the same surface of the portion of the mammal regardless ofany shape change which that portion of the mammal may experience duringrespiration.
 13. The device of claim 1 further comprising computationalmeans for receiving a signal representative of sensed volume of theportion of the mammal and for converting the signal to true volumevalues.
 14. A device for measuring the movement of an object,comprising: a. means for creating time-varying magnetic fields at leastlarge enough to surround the object, wherein the conductive coil meansare electrically connected in series so that the instantaneous volume Vmay be calculated from the voltage reading U of the volume output signalin the coil means which is receiving the induced voltage by use of theformulaV=U·d·k _(c) wherein d is the spacing between the coils; andk_(c)=a_(c)/U_(c); wherein a_(c) is the area of a reference coil, andU_(c) is the voltage reading of the volume signal when a calibrationcoil is attached; b. electrical circuits adapted to conform to thesurface of the object; and c. voltage monitoring means connected to theelectrical circuits, whereby motion of the surface creates a measurablechange in induced voltage in the circuits that correlates to themovement of the object, wherein the voltage monitoring means isconnected to a computing means suitable for performing a series ofalgorithmic steps to calculate the volume change of the object from themeasured induced voltage.
 15. The device of claim 14 in which theconductive coil means comprises a plurality of coil loops which areconfigured tightly about the thoracic region and the abdominal region ofthe mammal.
 16. The device of claim 14 further comprising sensing andcontrol means for controlling operation of the device, the sensing andcontrol means being electrically connected to the conductive coil means,the fixed coil means, and the current generating means.
 17. The deviceof claim 16 in which the sensing and control means comprises timing andmultiplex switching means for providing simultaneous volume measurementsof both a thoracic region and an abdominal region of the mammal.
 18. Thedevice of claim 17 in which the sensing and control means comprisesmultiplexing means for providing simultaneous measurement of a pluralityof regions of the mammal utilizing either phase, frequency or timemultiplexing.
 19. The device of claim 14 in which the current generatingmeans comprises a constant current circuit to maintain the current inthe conductive coil means constant regardless of the dynamic variationsof portions of the conductive coil means that are configured tightlyabout the various circumferences of at least one portion of the mammal.20. The device of claim 14 in which the current generating meanscomprises a signal generating and a constant current amplifierelectrically connected to the coil means which is receiving thegenerated circuit.
 21. The device of claim 14 in which the sensing andcontrol means comprises an amplifier and a rectifier electricallyconnected to the coil means that is receiving the induced voltage fromthe other coil means.
 22. A device for measuring the changing areawithin at least one portion of a mammal due to cardiac function,comprising: a. conductive coil means configured tightly about thevarious circumferences of at least one portion of the mammal, whereinthe conductive coil means comprises electrically conductive coil loopsthat are equally spaced on a flexible substrate that is suitable forwearing by the mammal, and further wherein said conductive coil loopsbeing connected in series so that a single voltage is measured, wherebymeasurement system complexity is substantially reduced; b. fixed coilmeans remotely located relative to the conductive coil means about themammal; and, c. current generating means for selectively providingalternating current to either one of the conductive coil means or thefixed coil means to create an induced voltage in the other coil meansrepresentative of true area within the coil means that is configuredtightly about the various circumferences of the mammal portion orportions, with the signals and area changing over time due to thecardiac function of the mammal.
 23. The device of claim 22 in which theconductive coil means comprises electrically conductive coil loops thatare closed circumferential loops.
 24. The device of claim 22 in whichthe current generating means generates current with a frequency range ofabout 10 kHz to about 200 kHz and from about 1 milliampere to about 1ampere.
 25. The device of claim 22 in which the conductive coil meanscomprises a plurality of coil loops which are configured tightly aboutthe thoracic region and the abdominal region of the mammal.
 26. Thedevice of claim 22 further comprising sensing and control means forcontrolling operation of the device, the sensing and control means beingelectrically connected to the conductive coil means, the fixed coilmeans, and the current generating means.
 27. The device of claim 26 inwhich the sensing and control means comprises timing and multiplexswitching means for providing simultaneous area measurements of both athoracic region and an abdominal region of the mammal.
 28. The device ofclaim 27 in which the sensing and control means comprises multiplexingmeans for providing simultaneous measurement of a plurality of regionsof the mammal utilizing either phase, frequency or time multiplexing.29. The device of claim 22 in which the current generating meanscomprises a constant current circuit to maintain the current in theconductive coil means constant regardless of the dynamic variations ofportions of the conductive coil means that are configured tightly aboutthe various circumferences of at least one portion of the mammal. 30.The device of claim 22 in which the current generating means comprises asignal generator and a constant current amplifier electrically connectedto the coil means which is receiving the generated current.
 31. Thedevice of claim 22 in which the sensing and control means comprises anamplifier and a rectifier electrically connected to the coil means thatis receiving the induced voltage from the other coil means.
 32. Thedevice of claim 22 in which the fixed coil means comprises a pluralityof small coil elements that are configured for matching and positioningto permit the fixed coil means to generate a magnetic field similar to asingle large coil for either sensing an induced voltage from theconductive coil means or for generating a field to create an inducedvoltage in the conductive coil means.
 33. The device of claim 32 inwhich the plurality of small coil elements comprises three small coilelements.
 34. The device of claim 33 in which each of the small coilelements is wound on a ferrite core and arranged linearly with optimizedpositions and signal intensity weighting to generate a homogeneousmagnetic field at the portion of the mammal being measured.
 35. Thedevice of claim 22 in which the conductive coil means compriseselectrically conductive coil loops that are equally spaced and carriedby an elastic and conformable substrate that is suitable for wearing bythe mammal in a manner similar to a tightly fitting garment which isconfigured so that the coil loops always conform to the same surface ofthe portion of the mammal regardless of any shape change which thatportion of the mammal may experience during cardiac function.
 36. Thedevice of claim 22 in which the conductive coil means compriseselectrically conductive coil loops that are spaced at constant and knownintervals and which are carried by an elastic and conformable substratethat is suitable for wearing by the mammal in a manner similar to atightly fitting garment which is configured so that the coil loopsalways conform to the same surface of the portion of the mammalregardless of any shape change which that portion of the mammal mayexperience during cardiac function.
 37. The device of claim 22 furthercomprising computational means for receiving a signal representative ofsensed area of the portion of the mammal and for converting the signalto true area values.
 38. A device for measuring the changing area withinat least one portion of a mammal due to cardiac function, comprising: a.conductive coil means configured tightly about the variouscircumferences of at least one portion of the mammal, wherein theconductive coil means comprises electrically conductive coil loops thatare equally spaced on a flexible substrate that is suitable for wearingby the mammal, wherein the conductive coil means are electricallyconnected in series so that the area A may be calculated from themeasured voltage U of a single coil loop which is receiving the inducedvoltage by use of the formulaA=U·k wherein k_(c)=a_(c)/U_(c); and wherein a_(c) is the area of areference coil, and U_(c) is the voltage reading of the volume signalwhen a calibration coil is attached; b. fixed coil means remotelylocated relative to the conductive coil means about the mammal; and, c.current generating means for selectively providing alternating currentto either one of the conductive coil means or the fixed coil means tocreate an induced voltage in the other coil means representative of truearea within the coil means that is configured tightly about the variouscircumferences of the mammal portion or portions, with the signals andarea changing over time due to the cardiac function of the mammal. 39.The device of claim 38 in which the conductive coil means comprises aplurality of coil loops which are configured tightly about the thoracicregion and the abdominal region of the mammal.
 40. The device of claim38 further comprising sensing and control means for controllingoperation of the device, the sensing and control means beingelectrically connected to the conductive coil means, the fixed coilmeans, and the current generating means.
 41. The device of claim 40 inwhich the sensing and control means comprises timing and multiplexswitching means for providing simultaneous volume measurements of both athoracic region and an abdominal region of the mammal.
 42. The device ofclaim 41 in which the sensing and control means comprises multiplexingmeans for providing simultaneous measurement of a plurality of regionsof the mammal utilizing either phase, frequency or time multiplexing.43. The device of claim 38 in which the current generating meanscomprises a constant current circuit to maintain the current in theconductive coil means constant regardless of the dynamic variations ofportions of the conductive coil means that are configured tightly aboutthe various circumferences of at least one portion of the mammal. 44.The device of claim 38 in which the current generating means comprises asignal generating and a constant current amplifier electricallyconnected to the coil means which is receiving the generated circuit.45. The device of claim 38 in which the sensing and control meanscomprises an amplifier and a rectifier electrically connected to thecoil means that is receiving the induced voltage from the other coilmeans.
 46. A device for measuring the changing area within at least oneportion of a mammal due to cardiac function, comprising: a. conductivecoil means configured tightly about the various circumferences of atleast one portion of the mammal, wherein the conductive coil means areelectrically connected in series so that the instantaneous volume V maybe calculated from the voltage reading U of the volume output signal inthe coil means which is receiving the induced voltage by use of theformulaV=U·d·k _(c) wherein d is the spacing between the coils; andk_(c)=a_(c)/U_(c); wherein a_(c) is the area of a reference coil, andU_(c) is the voltage reading of the volume signal when a calibrationcoil is attached; b. fixed coil means remotely located relative to theconductive coil means about the mammal; and c. current generating meansfor selectively providing alternating current to either one of theconductive coil means or the fixed coil means to create an inducedvoltage in the other coil means representative of true area within thecoil means that is configured tightly about the various circumferencesof the mammal portion or portions, with the signals and area changingover time due to the cardiac function of the mammal.
 47. A device formeasuring the changing area within at least one portion of a mammal dueto cardiac function, comprising: a. conductive coil means configuredtightly about the various circumferences of at least one portion of themammal, wherein the conductive coil means comprises electricallyconductive coil loops that are equally spaced on a flexible substratethat is suitable for wearing by the mammal, wherein the conductive coilmeans are electrically connected in series so that the area A may becalculated from the measured voltage U of a single coil loop which isreceiving the induced voltage by use of the formulaA=U·k wherein k_(c)=a_(c)/U_(c); and wherein a_(c) is the area of areference coil, and U_(c) is the voltage reading of the volume signalwhen a calibration coil is attached; b. fixed coil means remotelylocated relative to the conductive coil means about the mammal; and c.current generating means for selectively providing constant alternatingcurrent to either one of the conductive coil means or the fixed coilmeans to create an induced voltage in the other coil meansrepresentative of true area within the coil means that is configuredtightly about the various circumferences of the mammal portion orportions, with the signals and area changing over time due to thecardiac function of the mammal, said constant current substantiallyincreasing the accuracy of said measurement system.